How can I randomly generate a unit instance based on a probability?

Question

I want AirUnit to return a value between 1 and 3. I want LandUnit to return a value between 4 and 6. I want WaterUnit to return a value between 7 and 9.

The Array is suposed to be inside this function:

public function getRandom():Unit
{
    // Array
}

Answer 1

Choose a random number, and then write an if statement to return the appropriate new object based on the number.

I'm not an ActionScript coder so I don't know if this is correct, but generally it would look something like this:

// have an array called 'units' of three elements: an AirUnit, a LandUnit, a WaterUnit
random_num = Math.floor(Math.random()*9)+1; // random number from 1 to 9 inclusive

if(random_num <= 3) {
    return units[0]; // the AirUnit
} else if(random_num <= 6) {
    return units[1]; // the LandUnit
} else {
    return units[2]; // the WaterUnit
}

Again, that might not be exactly correct ActionScript, but that's the general code for what you seem to be asking for...

Answer 2

Your chances in your example are always even and therefore do not add any more randomness to the whole situation despite increasing their range, each choice always has the same number of chances to occur. For execution of speed you should just generate a random number of 0, 1 or 2 and use that as an index into the array to return the unit you want. The way most people generate random numbers between a 0 based range is to use a normal rand() call that returns a huge number and then use the modulus operator on it (the % symbol in most languages but I will admit I have not worked in action script in a very long time). So rand() % 3 would give you a number between 0 and 2.

If however what you are asking is for a way to change those values so you can say you will get a land unit 50% of the time, an air unit 30% of the time and a water unit 20% of the time, then the if statements mentioned by others could be changed to accommodate that sort of functionality. Just your original scenario does not show this.

Answer 3

I want to put WaterUnit, LandUnit and AirUnit in a Array and if the generated number of the array is 1-3 its a WaterUnit and if the generated number is 4-6 its a LandUnit and if the generated number is 7-9 its a AirUnit.

Your end result really has nothing to do with an array -- you just want to generate a random number between 1 and 9 (inclusive) and then, if that number is <= 3, return one type of object, otherwise if it is <= 6, return another type, et cetera.

In pseudo-code, that might look like this:

public function getRandom() : Unit {
  value = Math.RandomNumberBetween(1,9);
  if(value <= 3 ) {
    return new WaterUnit();
  } else if(value <= 6) {
    return new LandUnit();
  } else {
    return new AirUnit();
  }
}